Tantalum electrode for electric discharge devices



May 16, 1950 JENNE, JR 2,508,114

TANTALUM ELECTRODE FOR ELECTRIC DISCHARGE DEViCES Filed Dec. 5, 1947 Inventor Frank Jenn'e Jr.,

5 His Atior'ney.

Patented May 16, 1950 TANTALUM ELECTRODE FOB ELECTBI DISCHARGE Frank Jenne, In, Hoboken, N. 1., assignor to General Electric Company, a corporation of New York Application December 5, 1947, Serial No. 789,969

10 Claims. (Cl. 176128) This invention relates generally to gaseous electric discharge devices or lamps of the type employing ionizable mediums such as gases or vapors,and more particularly to improvements in electrode structures used therein.

In many discharge devices manufactured heretofore, the electrodes functioning as cathodes have been activated with materials of low workfunction, such as thoria or alkaline earth oxides. These electrodes provide ample electron emission at relatively low electrode temperatures and at low energy consumption for electrode heating. Furthermore, these electrodes also avoid electrode disintegration and envelope blackening from vaporization or sputtering of refractory electrode metal. The sputtering here referred to involves a knocking on of particles from an electrode by positive ions of the operating atmosphere that bombard the electrode under the impulsion of a high voltage drop existing adjacent the electrode while it is functioning as cathode-a cathode drop generally higher than the required ionizing potential of the atmosphere employed. Sputterin is especially apt to occur during the starting of an arc discharge.

Electrodes without activating oxides present difficulties which arise from the high work function of the electrode materials and from cathode hot-spotting, which is a concentration of the discharge and of the heating of the electrode at *a point or area so small that it has to be heated much hotter than adjacent areas in order to give emission corresponding to the discharge current.

The small hot-spot must yield the required enlission thermionically because field emission clue to the cathode drop is virtually nil from an unactivated electrode. Thus the hot-spot becomes heated to an excessive temperature at which point the electrode vaporizes rapidly or melts. This requires a high cathode drop which produces sputtering during starting and operation of the device, and contributes very little toward electrode heating or useful radiant output, thus producing a relatively poor overall efficiency.

In view of these difficulties, prior art discharge devices with unactivated electrodes have been used commercially in positions where end blackening or electrode disintegration was of secondary importance and where a higher initial starting voltage was tolerable.

An object of my invention is to provide a new and improved electrode structure for use in high pressure lamps.

Another object of my invention is to provide a 2 a new and improved electrode structure which facilitates starting of an electric discharge device.

A further object of my invention is to provide a new and improved electrode structure for an electric discharge device which will eliminate electrode disintegration during starting and will decrease the starting voltage thereby facilitating starting.

Further features and advantages of my invention will appear from the following description and species thereof. Fora better understanding of my invention reference may be made to the following description taken in connection with the accompanying drawings and its scope will be pointed out in the appended claims. Fig. l is a diagrammatic view of a discharge device or lamp, and a starting and operating circuit therefor incorporating my invention. Fig. 2 is an enlarged perspective view of the electrode structure 11- lustrated m Fig. 1 before it is secured to a leadin conductor. Fig. 3 is an enlarged perspective view of the electrode structure secured to a leadin conductor as illustrated in Fig. 1 embodying my invention. v

Referring to Fig. l, the discharge device I there illustrated is a lamp having an elongated vitreous radiation-transmitting discharge envelope 2, in the form of a tube of quartz or glass, provided with unactivated self-heating electrode assemblies 3 and 4 in its opposite ends, and permeable to ultraviolet and visible radiation. An auxiliary starting electrode assembly 5'is shown in one end of the envelope 2, closely adjacent the cooperating main electrode assembly 4. Besides an atmosphere of starting gas. such as one of the inert rare gases like argon, krypton, zenon, etc., the envelope 2 contains a vaporizable and ionizable working substance like mercury or other metal, to provide an operating atmosphere preferably at discharge-constricting pressure durin operation. The charge or working substance, represented :by a mercury droplet 6, may be more than will vaporize under the heat of the lamp, thus assurin operation with an atmosphere of saturated vapor; or it may be less than would afford an unvaporized surplus, so that the lamp will always operate with an' unsaturated atmosphere, as is now generally preferred for high-pressure lamps for some applications. In a device having the proportions hereinafter stated, argon at a pressure of 11 mm. of mercury is satisfactory for the starting gas. The envelope proportions illustrated represent a constriction and elongation of the device such that during operation on the rated discharge current a mercury pressure is developed which constricts the arc discharge into a narrow cord along the longitudinal axis of the envelope 2. As shown in Fig. 1, the envelope 2 may be a straight, uniform tube with molded ends somewhat reduced around the electrode assemblies 3 and 4 and having flat end walls from which greatly reduced necks I and 8 project around the main inlead wires 9 and i and the auxiliary inlead wire ll. These necks may embody graded seals, as here shown.

The electrode assemblies 3 and 4, and 5 are shown unactivated, consisting merely of bare refractory metal. The auxiliary starting electrode assembly 5 is of an ordinary form, comprising a simple straight piece of refractory metal wire, while the main operating electrode assemblies 3 and 4 (which may be counterparts of one another) are distinctive.

Illustrative circuit connections suitable for the starting and operation of my discharge device are shown in Fig. l as including a high-leakage reactance transformer i2 of semi-auto type with its primary connected across and A. C. power supply circuit l3 and with its secondary windings i4, i5 connected in series across the main discharge electrodes. Through a high currentlimiting resistance i6 and a thermal (bimetallic) switch i'l, one of the main electrode assemblies 4 and the associated auxiliary starting electrode assembly 5 are connected across the transformer secondary windings l4, [5 in parallel with the electrode assemblies 3 and 4. The heating resistor iii of the thermal switch I1 is shown connected in one side of the secondary circuit to the main electrode assemblies 3, 4 so as to be heated whenever the arc operates. For a discharge device intended to run on a current of 3 amperes at a voltage of 92.5 volts, the transformer may be so chosen as to produce a voltage of 230 volts across its serially connected secondaries l4, l5 on open circuit, and to give a secondary current of about 3.8 amperes on short-circuited.

Under these conditions, energization of the circuit It will automatically start the auxiliary discharge across the short electrode assembly gap 4, 5 and then the main discharge across the electrode assembly gap 3, 4. Thereafter the thermal switch II will disconnect the auxiliary electrode assembly 5 from the secondary circuit, so that the seal around the leads 9, II will not be injured by the D. C. voltage subsisting between them when both are in circuit; and this switch ll will remain open at all times when current is flowing.

Fig- 2 illustrates an enlarged perspective view of one of the'electrode structures of Fig. 1, for example electrode structure 3, before the electrode structure is secured to a lead-in conductor of the electric discharge device I.

Fig. 3 illustrates in more detail the electrode structure 3 of Fig. 2 secured to lead-in conductor l0. Electrode structure 3 comprises a hollow metallic cylinder which, for example, may be tantalum with one end thereof being slit to provide supporting means or integral extension arms is which converge for connection to leadin conductor I'Oand form inner concave surfaces normal to the length of the extension arms.

According -to my invention bifurcated or extension arms l9 serve as an arc initiatingmeans by providing a diverging path of travel for a glow discharge in the starting operation of device I. Generally speaking at a given voltage between two discharge surfaces a glow discharge is a steady-state self-sustaining discharge. That is, in a glow discharge the secondary electrons liberated per positive ion from the cathode or discharge surface must, in ionization by collision in some essential distance d. produce enough new electrons in the gas to maintain the discharge current at its constant value. This indicates that the cathode or discharge surface is an area of vital activity in sustaining a steady glow discharge, or in establishing a satisfactory glow discharge in the arc-initiating mechanism, and also that there is an important distance or length d from this area at which point ionization produced by secondary ions occurs. This correct distance it depends on many factors, two of them being the starting gas pressure in the electric discharge device and the type of material of the cathode surface. As a, glow discharge occurs and continues the gas in the area occupied by the glow discharge rarefies and allows the glow to expand in area.

The critical length d for the most desirably ionization operation of a glow discharge is difficult to obtain under high production manufacture of electric discharge devices wherein a glow discharge is used as a starting aid for striking an arc in the discharge device. Heretofore, unactivated hollow cylindrical electrodes have been used, but the proper cylinder diameter could not be obtained for the most favorable glow discharge operations because the area occupied by the glow would vary with different di charge devices and in each lamp with time needed for the starting operation.

Therefore, I provide an electrode structure which permits a glowdischarge to adjust itself for the most favorable operating condition. More particularly, I provide an electrode structure which provides a plurality of optimum spacings wherein a glow discharge may seek an area where secondary electrons liberated by the positive ions from the cathode produce enough new electrons in the gas to maintain the glow discharge current at its constant value. This critical distance d is maintained in the same device as the gas becomes rarefied in the glow discharge area. Thus, I provide a hollow cylindrical electrode having converging supporting means for serving as an arc initiating means by establishing a plurality of optimum spacings wherein a glow discharge may occur under the most favorable operating conditions and continue to remain under these most favorable conditions as the gas within the glow discharge area rarefies.

In addition, by providing the most favorable conditions for starting an arc discharge in an electric discharge device I have inherently reduced the starting voltage necessary. This in turn greatly reduces if not eliminates sputtering of the electrodes by means of a lower starting voltage and by proper ionization of the gas before a starting impulse is applied across the electrodes.

If desired, the starting electrode assembly 5 may be dispensed with. In this event a higher starting voltage would be required. My invention, however, is effective in reducing the starting voltage necessary for striking an arc discharge whether or not a starting electrode assembly is used.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In an electric discharge device of the type employing an ionizable medium capable of supporting an arc discharge the combination comprising an envelope, a lead-in conductor, and a self-heating thermionic electrode structure comprising a hollow metallic cylinder the axis of which is parallel to the arc discharge path and having integral rearwardly converging extensions joined together at their ends and connected to said conductor within the envelope, said extensions defining V-shaped arms serving as an arc initiating means by establishing a plurality of optimum spacings ranging from zero to the di- 2. In an electric discharge device of the type employing an ionizable medium capable of sup-' porting an arc discharge the combination comprising an' envelope, a single lead-in conductor, and a self-heating thermionic electrode structure comprising a hollow metallic cylinder having rearwardly converging integral extensions joined together at their ends and secured to said leadin conductor within said envelope for supporting said structure.

3. A self-heating thermionic electrode structure for an electric discharge device comprising a hollow metallic cylindrical electrode the rear end thereof being slit to provide converging arms joined together at their ends and serving as an arc initiating means by establishing a plurality of spacings.

4. A self-heating thermionic electrode structure for an electric discharge device comprising a hollow metallic cylindrical electrode having rearwardly converging integral extensions joined together at their ends and providing inner concave surfaces normal to the length of said extensions.

5. In an electric discharge device of the type employing an ionizable medium capable of supporting an arc discharge the combination comprising an envelope, a lead-in conductor, a start- 1118 electrode, and a self-heating thermionic electrode structure comprising a hollow metallic cylinder having integral rearwardly converging extensions joined together at their ends and connected to said conductor within the envelope, said extensions defining V-shaped arms serving with said starting electrode as an arc initiating means.

6. In an electric discharge. device of the type employing an ionizable medium capable of supporting an arc discharge the combination comprising an envelope, a lead-in conductor, a selfheating thermionic electrode structure comprising a hollow metallic cylinder the axis of which is parallel to the arc discharge path and having integral rearwardly converging extensions em- M8 lateral openings and Joined together at 'ameter of the cylindrical portion of the cathode.

their ends and connected to said conductor within the envelope, said extensions defining V- shaped arms serving as an arc initiating means, and a starting electrode positioned next to said structure for initiating a glow discharge within said converging extensions.

7. In an electric discharge device of the type employing an ionizable medium capable of supporting an arc discharge the combination comprising an envelope, a lead-in conductor, and a self-heating thermionic electrode structure comprising a hollow tantalum electrode having integral rearwardly converging extensions joined together at their ends and connected to said conductor within the envelope, said extensions defining V-shaped arms serving as an arc initiating means by establishing a plurality of optimum spacings for establishing a, glow discharge.

8. In an electric discharge device of the type employing an ionizable medium capable of supporting an arc discharge the combination comprising an envelope, a lead-in conductor, and a self-heating thermionic electrode structure comprising a hollow tantalum electrode having rearwardly converging integral extensions joined together at their ends secured to said lead-in conductor within the envelope for supporting said structure.

9. An unactivated self-heating thermionic electrode structure for an electric discharge device comprising a hollow tantalum cylindrical electrode the rear end thereof being slit to provide converging arms joined together at their ends for serving as an arc initiating means by establishing a plurality of spacings.

10. An unactivated self-heating thermionic electrode structure for an electric discharge device comprising a hollow tantalum cylindricalelectrode having rearwardly converging extensions joined together at their ends and providing inner concave surfaces normal to the length of said extensions.

FRANK JENNE, Jn.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATEN'I'S Number Name Date 1,852,020 Metcalf Apr. 5, 1932 V 1,877,716 Claude Sept. 13, 1932 1,922,244 Hunter Aug. 15, 1933 2,267,318 Aicher Dec. 23, 1941 

1. IN AN ELECTRIC DISCHARGE DEVICE OF THE TYPE EMPLOYING AN IONIZABLE MEDIUM CAPABLE OF SUPPORTING AN ARC DISCHARGE THE COMBINATION COMPRISING AN ENVELOPE, A LEAD-IN CONDUCTOR, AND A SELF-HEATING THERMIONIC ELECTRODE STRUCTURE COMPRISING A HOLLOW METALLIC CYLINDER THE AXIS OF WHICH IS PARALLEL TO THE ARC DISCHARGE PATH AND HAVING INTEGRAL REARWARDLY CONVERGING EXTEN- 